High-strength steel sheet and production method for same, and production method for high-strength galvanized steel sheet

Abstract

Disclosed is a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more and excellent in ductility, fatigue properties, stretch flangeability, surface characteristics, and sheet passage ability that can be obtained by providing a predetermined chemical composition and a steel microstructure that contains, by area, 20-50% of ferrite, 5-25% of bainitic ferrite, 1-10% of martensite, and 5-15% of tempered martensite, and that contains, by volume, 10% or more of retained austenite, in which the retained austenite has a mean grain size of 2 μm or less, a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, the retained austenite has a mean free path of 1.2 μm or less, and the tempered martensite has a mean free path of 1.2 μm or less.

The invention claimed is: 
     
       1. A high-strength steel sheet comprising:
 a chemical composition containing, in mass %, C: 0.08% or more and 0.35% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.50% or more and 3.00% or less, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, and N: 0.0005% or more and 0.0100% or less, and optionally at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Ti: 0.005% or more and 0.100% or less, Nb: 0.005% or more and 0.100% or less, B: 0.0001% or more and 0.0050% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.0020% or more and 0.2000% or less, Sn: 0.0020% or more and 0.2000% or less, Ta: 0.0010% or more and 0.1000% or less, Ca: 0.0003% or more and 0.0050% or less, Mg: 0.0003% or more and 0.0050% or less, and REM: 0.0003% or more and 0.0050% or less, and the balance consisting of Fe and incidental impurities; and 
 a steel microstructure that contains, by area, 20% or more and 50% or less of ferrite, 5% or more and 25% or less of bainitic ferrite, 1% or more and 10% or less of martensite, and 5% or more and 15% or less of tempered martensite, and by volume, 10% or more of retained austenite, wherein 
 the retained austenite has a mean grain size of 2 μm or less, 
 a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, 
 the retained austenite has a mean free path of 1.2 μm or less, and 
 the tempered martensite has a mean free path of 1.2 μm or less, and
 wherein the high-strength steel sheet has a tensile strength (TS) of 780 MPa or more and satisfies: 
 total elongation (EL)≥34% for TS 780 MPa grade, EL≥27% for TS 980 MPa grade, and EL≥23% for TS 1180 MPa grade, 
 stretch flangeability (λ)≥40% for TS 780 MPa grade, λ≥30% for TS 980 MPa grade, and λ≥20% for TS 1180 MPa grade, 
 fatigue limit strength≥400 MPa, and 
 fatigue ratio≥0.40. 
 
 
     
     
       2. A production method for a high-strength steel sheet, the method comprising:
 heating a steel slab having the chemical composition as recited in  claims 1  to 1100° C. or higher and 1300° C. or lower; 
 hot rolling the steel slab with a finisher delivery temperature of 800° C. or higher and 1000° C. or lower to obtain a steel sheet; 
 coiling the steel sheet at a mean coiling temperature of 450° C. or higher and 700° C. or lower; 
 subjecting the steel sheet to pickling treatment; 
 optionally, retaining the steel sheet at a temperature of 450° C. or higher and Ac 1  transformation temperature or lower for 900 s or more and 36000 s or less, 
 cold rolling the steel sheet at a rolling reduction of 30% or more; 
 subjecting the steel sheet to first annealing treatment whereby the steel sheet is heated to a temperature of 820° C. or higher and 950° C. or lower; 
 cooling the steel sheet to a first cooling stop temperature at or below Ms at a mean cooling rate to 500° C. of 15° C./s or higher; 
 subjecting the steel sheet to second annealing treatment whereby the steel sheet is reheated to a temperature of 740° C. or higher and 840° C. or lower; 
 cooling the steel sheet to a cooling stop temperature of 150° C. or higher and 350° C. or lower at a cooling rate of 1° C./s or higher and 15° C./s or lower; and 
 reheating the steel sheet to a reheating temperature of higher than 350° C. and 550° C. or lower and retaining the steel sheet at the reheating temperature for 10 s or more, to produce the high-strength steel sheet as recited in  claim 1 . 
 
     
     
       3. A production method for a high-strength galvanized steel sheet, the method comprising subjecting the high-strength steel sheet as recited in  claim 1  to galvanizing treatment.